Damselfish neurofibromatosis (DNF) is a transmissible disease involving neurofibromas and chromatophoromas affecting bicolor damselfish Stegastes partitus on Florida reefs. Analysis of genomic DNA by Southern blotting techniques demonstrated the presence of a group of extrachromosomal DNAs in tumors from fish affected with DNF but not in healthy individuals. Cell lines obtained from tumors contained identical DNAs and were shown to be tumorigenic in vivo, while lines established from healthy fish did not contain such DNA and were not tumorigenic. These DNA patterns were also observed in experimentally induced tumors. A DNase resistant component of this DNA was isolated from both tumor cells and conditioned media of tumor cell lines suggesting that these sequences were encapsulated in viral particles. These data support the hypothesis that one or more of these extrachromosomal DNA forms is the genome of an unusual virus and that this virus is the etiologic agent of DNF. We have tentatively termed this agent the damselfish virus-like agent (DVLA).